High Energy Efficiency Retrofits in Portugal


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1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of KES International doi: 10.1016/j.egypro.2015.12.209

Energy Procedia 83 ( 2015 ) 187 – 196


7th International Conference on Sustainability in Energy and Buildings

High energy efficiency retrofits in Portugal

Fernanda Rodrigues


, Marlene Parada


, Romeu Vicente


, Rui Oliveira


and Ana Alves


aCivil Engineering Department, University of Aveiro, Campus Universitário Santiago 3810-193, Aveiro, Portugal


The current European concerns for new and existent buildings are currently being tackled as "Near-Zero Energy Buildings" in the scope of the Energy Performance of Building Directive (EPBD, 2010).

Buildings’ retrofit is taken as a valuable and sustainable opportunity for reducing energy consumption and greenhouse gases (GHG) emissions in order to accomplish the nZEB requirements. In Portugal, the complexity associated to current renovation processes carried out within urban areas usually leads to unsustainable and inefficient energy strategies, being crucial therefore, to raise awareness for the development of more reliable retrofit solutions to invert this trend..

The high energy performance of buildings achieved by complying with the requirements of Passive House standard is subjacent in EPBD. Energy retrofit based on passive solutions will contribute to the sustainability of urban centres on different levels: environment, economic and social.

Hence, this study aims to contribute for the exploitation of this particular matter by assessing the favourable applicability of EnerPHit standard in existing buildings, taking for case study a 19th Century stone masonry building, located in Oporto, in Portugal, which was subjected to a deep retrofit process.

© 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of KES International.

Keywords: Energy; refurbishement; EnerPHit; nZEB; efficiency.

1. Introduction

Nowadays, and despite the building sector considered as a major energy consumption and producer of CO2

emissions, according to the Intergovernmental Panel on Climate Change [1], this sector represents the largest untapped source of cost-effective energy saving and greenhouse gases (GHG) reduction potential within Europe. To overcome the underinvestment and the complexity associated to the incorporation of efficient energetic strategies to renovation processes of existing buildings, the 2012 Energy Efficiency Directive [2] has raise awareness to this matter by stating that all Member States (MS) should develop long-term renovation strategies suitable to their building environment and reality. More recently, the European Parliament through the recast of the Energy Performance of Building Directive (EPBD) placed nZEB as future goal, whereas by definition, “the nearly zero or © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of KES International


very low amount of energy required should, to a very significant extent, be covered by energy from renewable sources, including renewable energy produced on-site or nearby” [3]. According to EPBD’s requirements, each Member State (MS) must define their methodologies in function of their national and local conditions both for new and existing construction. To accomplish these requirements several MS have being adopting performance-based solutions and prescriptive criteria that were recently embedded in their own national building codes [4].

Moreover, according to IPCC, buildings represent a critical portion of a low carbon future and a global challenge for integration with sustainable development [5]. Buildings embody the biggest need for energy, especially in developing countries, while much of the existing energy used in buildings is very wasteful and inefficient. So, existing and future buildings will determine a large portion of global energy demand. Despite current trends indicate the potential for massive increases in energy demand and associated emissions, buildings offer cost effective opportunities to reduce energy demand, contributing at the same time for the enhancement at other key sustainable development levels including poverty alleviation, energy security, and improved employment conditions.

Nowadays it is believed that buildings’ renovation according with high energy performance standards could be one of the most cost effective investments a nation can make, given the benefits in terms of job creation, quality of life, fostering of economy, climate change mitigation and energy security. Particularly in developed countries, the massive existing building stock is being renewed very slowly, being therefore of major relevance the introduction of specifically directed policies to act over the existing building stock, which are expected to increase the reliability of these renovations processes in terms of energy performance. These policies should address every aspect of design, construction, and operation of buildings throughout the world, bearing in mind the significant advances in building technology, codes and appliance standards in European countries, which are focused on reducing the current total building energy use trends [5]. Among many other mitigation strategies pointed out concerning energy consumption and GHG emissions of the building sector, IPCC highlights the high performance building envelope and efficient appliances (lighting, heating, ventilation), both integrated in the concept of Passive House.

In this sense, this paper aims to contribute to the high energy retrofit of old buildings, depicting the results of a case study supported on a XIX Century building, which renovation design was modified to comply with the EnerPHit standard.

2. High energy efficient buildings

The current reduction needs of buildings’ energy use led worldwide to pursuit the goal of low and very low energy buildings, thus appearing different definitions and concepts. Despite these differences all have in common that very low energy houses have a design that enables low energy demand through: well insulated building envelope for minimal heat losses; compact shape and no thermal bridges for minimal heat losses; energy efficient windows facing sun allowing use of passive solar gains; good airtightness for controlled ventilation and reduction of heat losses and moisture damages [6]. Hence, these buildings have significant lower energy demand than buildings just meeting the mandatory buildings regulations, which typical criteria are 25-50% better than minimum requirements [6]. Besides the obvious benefit of low energy consumption, the very low energy house has many benefits in regard to the comfort and the indoor climate (relates to the air temperature, the mean radiant temperature, the air velocity, the humidity and the activity and clothing of the persons in the interior).

A number of voluntary standards for heating energy demand aiming high comfort with minimum consumption have been developed in various countries for residential and non-residential buildings, such as the Passive House standard.

2.1. Passive House

The concept of Passive House has been developed in Germany by the Passive House Institute and is currently considered the most demanding standard on buildings’ energy efficiency [7]. A passive house’s annual heating demand must be equal or less than 15 kWh.m-2.y−1 (assuming a uniform indoor temperature of 20ºC) or, the heating load must be equal or less than 10 W.m−2, the primary energy use must be equal or less than 120 kWhm−2 y−1, the airtightness level (n50) must be equal or less than 0.60 h−1 and thermal comfort must be met for all living areas


overheating). To achieve these requirements it entails a high performance thermal envelope combined with mechanical ventilation with heat recovery to ensure high indoor air quality. Thus, the building envelope has to comply with several thermal requirements for Central European countries, such as very well-insulated opaque building components and efficient mechanical ventilation with heat recovery. For the most cold climates, this means a maximum value of 0.15 W/(m²K) for the heat transfer coefficient, U-value. Moreover, window frames must be well insulated, which means a maximum U-value of 0.80 W/(m²K) and g-values around 50%, again for the most cool climates. Finally, uncontrolled leakage through gaps must be smaller than 0.6 of the total house volume per hour during a pressure test of 50 Pascal (both pressurized and depressurized) and the absence of thermal bridges should be guaranteed [7].

Passive House standard represents a reduction factor up to 12 for heating load in mild climates (such as Southern Europe) and up to 30 for cold climate regions with minimal insulation requirements. In cases where buildings are not currently heated up to comfortable temperatures, the adoption of a high performance envelope can aid in achieving comfortable conditions while still reducing heating energy use in absolute terms [6].

Considering the cooling season, cooling energy use is growing rapidly in many regions where, with proper attention to useful components of vernacular design combined with modern passive design principles, mechanical air conditioning would not be needed. This use includes regions with a strong diurnal temperature variation, in which a combination of external insulation, exposed interior thermal mass, and night ventilation can maintain comfortable conditions, a strong seasonal temperature variation, whereas the ground can be used to cool incoming ventilation air or even dry regions, allowing evaporative cooling or hybrid evaporative/mechanical cooling strategies to be implemented [6].

Combining insulation levels that fulfill the Passive House standards for heating demand in Southern Europe with the above mentioned strategies, heating loads can be reduced by a factor of 6–12, from 100-200 kWh.m-2.y-1 to 10-15 kWh.m-2.y-1 and cooling loads by a factor of 10, from 30 kWh.m-2.y-1 to 3 kWh.m-2.y-1 [8].

For existing buildings the EnerPHit standard were developed by limiting the annual heating demand to a maximum of 25 kWh.m-2.y-1, assuming a uniform indoor temperature of 20ºC. Moreover, while the primary energy demand is taken equal or lower than 120 kWh.m-2.y-1 + (QH – 15 kWh.m-2.y-1 x 1.2), the maximum value of the air

tightness level (n50) is considered equal to 1.0 h−1 [7]. In Germany, more than 13,000 Passive Houses have been built

since the 1990s [9 cit. by 10] and the energy renovation according to the EnerPHit standard are growing [7], being considered by some authors as one of the fastest growing energy performance standard in the world [11].

3. Building retrofit according EnerPHit 3.1. Case study

The thermal performance of a XIX Century building was studied according with EnerPHit standard aiming to show the optimization that has been done during the design phase of its renovation process in order to meet the Passive House standards.

The building under study is situated in the historical centre of Oporto (latitude 41°08', longitude 8°36'), characterized by a heating degree-hours equal to 38 kKh.y-1 and the main facade is North orientated (see Fig. 2a). This architectural valued building comprises three elevated floors of residential use and the ground floor level for commercial use, with facades covered by ceramic tiles of great patrimonial value and other external architectonic features that have to be preserved and maintained. Thus, the insulation layer has to be executed on the internal side of external walls. The thickness of granite stone walls varies from 25 cm to 65 cm, for external and partition walls. The interior floors and ceilings are in timber structure.

As a first task, the performance of the original building according to the thermal balance of the Passive House concept was studied. In the scope of the Portuguese Thermal Code for Residential Buildings (REH), the thermal balance is obtained for each individual flat (Fig. 1a), while by the Passive House concept the thermal envelope considers only one internal volume that in this case study involve the three flats (Fig. 2b). The thermal balance was obtained through the calculation application Passive House Planning Package – PHPP8. In this sense, for these three dwellings with a treated floor area (TFA) of 120.6 m2, considered as one thermal volume, and without considering the inclusion of any insulation measure, the following preliminary results concerning the energy demands were


obtained through the PHPP8: heating demand of 145 kWh.m-2.y-1; heating load of 58 W.m-2 and primary energy demand of 304 kWh.m-2.y-1.

a) b)

Fig. 1. (a) Thermal envelope: Passive House standard; (b) Thermal envelope: Portuguese Thermal Code. 3.2. Optimization of the building

In order to accomplish the Portuguese thermal requirements, the designer has decided to adapt 4 cm of XPS on the internal face of the stone envelope (external walls), 5 cm in the 11 cm thick brick wall between the dwellings and the staircases, and two layers with 3 cm and 5 cm of XPS in the floor above the commercial space and in the ceiling of the third floor (above this is an unheated space – a loft). The thermal characteristics of each envelope component are detailed in Fig. 2b. According to the designer’s measurements, it was not possible to comply with the EnerPHit standard, as demonstrated in Table 1.

South and North elevation Floor plan Exterior Wall Interior Wall (stairs)

3D Model XIX Century Building

Ceiling/Floor slab 1 – Plasterboard (2 cm) 2 – Thermal insulation XPS (4 cm) 3 – Stone wall 4 – Render (2 cm) 5 – Thermal insulation XPS (5 cm) 6 – Brick wall (11 cm) 7 – Interior coating (0.5 cm) 8 – Floor screed (7 cm) 9 – Neoprene layer (1 cm) 10 – Thermal insulation XPS (3 cm) 11 – OSB panels (1.8 cm) 12 – Wooden structure (3 cm) 13 – Air space (15 cm)

Fig. 2. (a) Architecture plans; (b) Constructive solutions.

To achieve the Passive House requirements, according to the EnerPHit, a ventilation system with heat recovery (MVHR) is needed to guarantee the housing thermal comfort. The heat recovery system, composed by a compact


MVHR – Mechanical Ventilation with Heat Recovery


heat pump with 80% efficiency and a storage mass integrated to produce the domestic hot water, was located in the loft, outside the thermal envelope (Fig. 1a). This central ventilation system was designed to supply fresh air into the main rooms and to extract air from kitchens and toilets, taking advantage from the cross-ventilation principle, as showed in Fig 3a. Therefore, to optimize the ventilation system, flats were divided into two zones (supply air zone and extract air zone), maximising the air exchange in all the compartments through the air renovation from the supply to the extract zone and avoiding the air dispersion between compartments.

Taking advantage of the roof’s favourable solar exposure, solar panels were considered to heat domestic water. The solar system was composed of a 6 m2 area of solar collector and a stratified solar storage tank with 300 liters of capacity. As depicted in Fig 3b, during almost the whole year there is a high solar radiance incidence, from which results high solar energy recovery indexes, allowing to efficiently heat domestic water only by means of solar energy, contributing this way to reduce the heating demand.


O - Extract air

O - Supply air b)

Fig. 3. (a) Layout of the ventilation network; (b) Annual solar radiation in Oporto.

The performance of the MVHR was assured through the envelope airtightness. In this building, even though stone walls of the external envelope guarantee the air tightness, the timber floor of the first level and the ceiling of the third had to be treated with air-tightness tapes and membranes to achieve the maximum air change rate value of n50 ≤ 0.6 h-1. It was also considered the sealing of the windows frames with air-tightness tapes.

In which regards the insulation of the envelope, to achieve the Passive House requirements according the EnerPHit standard, 10 cm of XPS was needed on the walls, floors and ceilings (Table 1). The minimization of thermal bridges (analyzed with THERM7®), which is important to avoid heat losses through the envelope, was found unnecessary due to this particular type of constructive process.

As the treated floor area of the dwellings is very small, the internal insulation leads to a significant reduction of this area. Thus, the results obtained from the application of VIP (vacuum insulation panels - a microporous insulation material with the advantages of vacuum insulation technology) were analyzed and it was concluded that with only 2 cm of VIP panels (with a coefficient of conductivity (ʎ)=0.007 W.m-1.K-1) the EnerPHit standard are fulfilled (see Table 1).

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 200 400 600 800 1000 1200 1400

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Sol a r Fr act ion [ -] Sol a r r a di at ion, heat ing dem a nd [k W h/ m ont h] Heat demand

Heat demand covered by solar Radiation on tilted collector surface

Directed air flow


Table 1. Thermal characteristics and thermal balance results Energy retrofit according to Portuguese requirements

Energy retrofit according to EnerPHit requirements

Energy retrofit according to EnerPHit requirements (VIP) Construction type Stone walls + wooden floors Stone walls + wooden floors Stone walls + wooden floors

Treated floor area, m2 120.6 120.6 120.6

UFloor slab ,W.m-2.K-1 0.270 (XPS = 8 cm ) 0.236 (XPS = 10 cm) 0.227 (VIP = 2 cm) UWall,W/(m2K) 0.519-0.711 (XPS = 5-4 cm) 0.305–0.331 (XPS = 10 cm) 0.291–0.315 (VIP = 2 cm) UCeiling_Loft,W. m-2.K-1 0.281 (XPS = 8 cm) 0.244 (XPS = 10 cm) 0.235 (VIP = 2 cm) UWindow,W. m-2.K-1

2.500 (timber window double glazing)

1.670-2.000 (timber double low-e glazing)

1.670-2.000 (timber double low-e glazing)

Ventilation system HVAC Mechanical ventilation with heat


Mechanical ventilation with heat recovery

Night ventilation Yes Yes Yes

Heating/Cooling generation Individual HVAC unities (air to air)

Compact heat pump (air to air) Compact heat pump (air to air)

DHW source Boiler Compact heat pump and solar


Compact heat pump and solar thermal

Lighting type CFLs LED LED

n 50 a, h-1 0.60 0.60 0.60

Heating demand b, kWh.m-2.y-1 81.30 23.50 22.70

Cooling demand b, kWh.m-2.y-1 0.00 0.00 0.00

Overheating, in % b 0.00 0.00 0.00

Primary energy b (Max.Value), kWh.m-2.y-1

214.10 (200.00) 102.10 (130.00) 99.80 (129.00)

a maximum value used in PHPP8. b According to PHPP8 calculations.

3.3. Different climatic zones – parametric study

This building was studied for six climatic zones of cities located in the North and Centre of mainland Portugal, to which this constructive typology is frequently observed. Hence, the following cities of Bragança, Vila Real, Braga, Guarda, Viseu and Viana do Castelo were chosen, comprehending heating degree hours of 38, 36, 40, 58, 33 and 40 kKha-1, respectively.

Morover, Table 2 shows the constructive solutions that have fulfilled the EnerPHit requirements for a maximum heating demand of 25 kWh.m-2.y-1 for each city. The building located in Oporto was considered as the reference (solution 1), varying the insulation thickness (with XPS or VIP) of the constructive elements and the glazing and windows frame type, and maintaining the MVHR system (that includes the hot water heating system too).

To achieve the requirements of the EnerPHit standard different solutions were considered according to these cities: solutions 2 and B for the cities of Bragança, Vila Real, Braga and Viana do Castelo; solutions 1 and A for Viseu and the solutions 3 and C for Guarda. These solutions are in accordance with the severity of the climatic conditions for each city during the heating season, represented by the previously mentioned heating degree hour’s values. Beyond large insulation thickness it was also required triple glazing windows for most of the solutions. Even though these types of solutions are not usually required in the scope of the current Portuguese Thermal Code (REH), they are considered fundamental to comply with the high energy efficient standard under study.


Table 2. Thermal characteristics of the solution in each city studied, considering a maximum air change rate value, n50=0.6 h-1.

Type of insulation XPS VIP

Solution/Features of the building 1 2 3 A B C Thermal insulation of walls (cm) 10 10 15 2 1.5 3 Thermal insulation of floor and ceiling (cm)

10 10 15 2 1.5 3

Window Double glazinga Triple glazingb Triple glazingb Double glazinga Triple glazingb Triple glazingb a U

f = 1.60 W/(m2K); Ug = 1.30 W/(m2K); gT = 0.64. b U

f = 0.81 W/(m2K); Ug = 0.65 W/(m2K); gT = 0.60.

Fig. 4a and 4b depict the results obtained for the different cities under study, from which is possible to conclude that the city of Guarda is the one presenting the highest energy demand in order to achieve the minimum values required by the EnerPHit standard. With the same constructive solution (solution 2 or B) the EnerPHit requirements were achieved for the majority of the cities (Bragança, Vila Real, Braga and Viana do Castelo) being possible to establish a standard solution for these different climatic zones. For Viseu the result of the energy balance is similar to that latter, however it was achieved with double-glazing windows, XPS thermal insulation (solution 1) and thinner VIP insulation (solution A).

Fig. 4. (a) Energy demand for the studied cities with XPS insulation; (b) Energy demand for the studied cities with VIP insulation. Considering the internal comfort during cooling season, Braga and Viana do Castelo have no overheating problems displaying values under 4%. Despite of being a building with thick stone walls, Bragança, Vila Real, Viseu and Guarda have overheating values near the maximum limit of 10%, due to its hotter/warmer summers. With respect to the thermal balance, a natural night ventilation strategy was simulated by opening the windows in both facades, North and South, promoting this way the cross ventilation and the passive cooling. Notwithstanding of the building’s small south glazing areas Bragança, Viseu and Guarda present significant overheating values (according with Fig. 4 (a) and Fig. 4 (b) they are still below the maximum admissible value). These values easily can be decreased by improving the indoor comfort during the cooling season by means of introducing interior shading devices such like as shutters.

However all the analyses carried out to the mentioned climatic zones did not exceed the maximum value established by the EnerPHit standard for primary energy, in the city of Guarda this value is very close to this limit. During this study, the heating load analysis was disregarded/neglected since EnerPHit standard does not establish a limit for this parameter.

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Bragança Vila Real Braga Guarda Viseu Viana do Castelo Annual Heating demand (kWh.mˉ².y¯¹) Primary energy demand (kWh.mˉ².y¯¹) Overheating (%)

Limit value of the Primary Energy (kWh.mˉ².y¯¹) Limit value of the Annual heating demand (kWh.mˉ².y¯¹)

Limit of the overheating (%) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Bragança Vila Real Braga Guarda Viseu Viana do Castelo Annual Heating demand (kWh.mˉ².y¯¹) Primary energy demand (kWh.mˉ².y¯¹) Overheating (%)

Limit value of the Primary Energy (kWh.mˉ².y¯¹) Limit value of the Annual heating demand (kWh.mˉ².y¯¹) Limit of the overheating (%)


3.4. Economic viability of the EnerPHit building

The energy retrofit of the heritage building according to this standard led to an additional investment and subsequent economic analysis, carried out in order to determine its payback period in the different locations. Thus, the heating energy demands of the renovated building were considered taking into account both requirements of the Portuguese Thermal Code and the EnerPHit. The method of Net Present Value (NPV) was applied for the investment evaluation. This method consists on the summing of all the net cash flows, i.e. the difference between the revenue and expenditure during the project, indicated in Equation 1 [12], enabling the assessment of the viability of the project.



t t t







where NPV is the net present value, NCFt is the net cash flow generated by innovation project in year t and r is the

discount rate. While the gas consumption was computed by considering a price of 0.079 €/kWh and the annual inflation rate of 2.50 %, the electricity consumption was computed considering a price of 0.155 €/kWh and a corresponding annual inflation rate of 2.80 %.

Subsequently the energy savings from the first day of operation of the house built according the EnerPHit standards and according to the Portuguese Thermal Code’s requirements were obtained. The payback period is attained when the difference between savings accumulated over the years and the additional investment inflated with an interest rate of 3.75%.y-1 equals zero.

For Oporto (1st location) it can be concluded that the overall costs for the EnerPHit renovation were evaluated 6.16% (1559.50 €/m2) higher than the solution according to the Portuguese Thermal Code requirements for XPS solution, representing an 11-year payback period. With respect to the use of the VIP insulation solution the overall cost was evaluated in 21.39% (1861.70 €/m2) higher, to which corresponds a payback over 30 years. For the city of Viseu, the results of this economic analysis are similar to the latter ones for constructive solutions 1 and A.

For climatic zones of Bragança, Braga, Vila Real and Viana do Castelo (2nd location) it was concluded that the renovation overall costs were evaluated 6.79% (1570.00 €/m2) higher than the solution according to the Portuguese Thermal Code requirements for XPS solution leading to a payback of 12 years. With respect to the use of VIP insulation solution, the overall costs were evaluated in 20.30 % (1836.18 €/m2) higher and the payback is over 30 years.

For the most unfavorable situation, the city of Guarda (3rd location), it was concluded that the renovation overall costs were estimated 8.79% (1604.46 €/m2) higher than the solution according with the Portuguese Thermal Code requirements for XPS solution leading to a payback of 11 years. With respect to the use of VIP insulation solution the overall costs were found 26.06 % (1979.20 €/m2) higher and the payback is also over 30 years (see Table 3).

The average value of 7% for over cost and the consequent payback period up to 11-12 years is reasonable when considering the XPS solution. However, the VIP solutions, ideal for inner side insulation, were found not yet economically viable, taking into consideration its thickness. In future, with the increase of its application, this insulation solution might be more competitive comparing with the common insulation materials.

Therefore, the over cost of the initial investment in the building renovation leads to low energy consumption throughout the buildings’ life cycle associated with an excellent thermal comfort and indoor air quality, which value cannot be measurable.


Table 3. The results of the economic study for three climatic locations

XPS solution VIP solution

Situation 1st location 2nd location 3rd location 1st location 2nd location 3rd location Cost per m2 (€/m2) 1559.50 1570.00 1604.46 1861.70 1836.18 1979.20 Additional investment (%) 6.16 6.79 8.79 21.39 20.30 26.06 Payback period (y) 11 12 11 > 30 > 30 > 30 4. Conclusions

The EPBD was transposed for the Portuguese legal framework and came into force on the 1st December 2013 with the publication of the Thermal Code for Residential Buildings (REH) and for Commercial and Service Buildings (RECS) [14] in the same law. For the residential buildings this Code define the demanding requirements and Energy performance based on buildings’ thermal performance and on the energy efficiency of the respective technical systems (domestic hot water, cooling, heating, ventilation, lighting). It also promotes the renewable energy use and has a roadmap with progressive higher thermal requirements for the building envelope aiming to achieve the near zero energy building concept. Even though these requirements are approaching the Passive House’s opaque envelope requirements, the limits for both heating and cooling energy demand and for primary energy consumption are far from being intended and rooted such as they are on acclaimed PH standard. The high energy efficiency standard for energy retrofit, the EnerPHit standard, clearly defines the requirements for buildings’ retrofit for annual heating demand, for primary energy demand and for air change rate.

The energy retrofit of existent buildings, which patrimonial value impose the external finishing maintenance, is more demanding because internal insulation use leads to thermal inertia loss and consequently to a lower energy performance. By other hand the use of internal insulation with thicker layers is also difficult considering the architectural and geometric features of internal constructive elements, and also the reduced internal areas of these old buildings. The building under study has no restriction about internal finishing maintenance allowing the use of internal insulation. To avoid eventual constructive issues and the reduction of the internal area, the use of thin insulation material with high thermal performance (like VIP) is likely to be the optimal solution. However, its current expensive cost does not make its application sustainable from the economical point of view. The payback period achieved for the VIP solution (over 30 years) demonstrates a negative cost-benefit relation. Despite the restrictions that should be overcome during the energy retrofit of architectural valued buildings, the parametric study conducted to this XIX Century stone masonry building according to the EnerPHit standard, concluded that it is possible to achieve high thermal performances complying with those demands. Hence, it is necessary to develop national requirements concerning high efficiency retrofit that can be suitable to the massive existing building stock in need of major refurbishment actions, and thus effectively contributing to the reduction of energy consumption and GHG emissions.

The over investment registered to achieve the mentioned demands is related to thicker insulation layers, higher performance of the glazing areas and to the detail that is needed during the construction phase to obtain an airtight envelope. The planning, the supervision and the quality of the manpower are essential factors to effectively meet the PH standard. Finally, all these principles will work together on contributing for the accomplishment of low energy and low carbon houses with higher durability.

Acknowledgements: The authors acknowledge architects Pedro Resende Leão, Ana Maria Carvalho and Diana



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